1. Field of the Invention
The present invention relates to an ammoxidation catalyst for use in producing acrylonitrile or methacrylonitrile from propane or isobutane by ammoxidation in the gaseous phase. More particularly, the present invention is concerned with an ammoxidation catalyst comprising a compound oxide which contains, in specific atomic ratios, molybdenum; vanadium; niobium; at least one element selected from tellurium and antimony; and at least one element selected from ytterbium, dysprosium, erbium, neodymium, samarium, lanthanum, praseodymium, europium, gadolinium, terbium, holmium, thulium, lutetium and scandium. By the use of the ammoxidation catalyst of the present invention, the ammonia-based yield of acrylonitrile or methacrylonitrile can be largely increased without sacrificing the propane- or isobutane-based yield of acrylonitrile or methacrylonitrile. In other words, in the present invention, the feedstock ammonia can be efficiently utilized in the ammoxidation of propane or isobutane while achieving an efficient utilization of propane or isobutane. The present invention is also concerned with a process for producing acrylonitrile or methacrylonitrile by using such an excellent ammoxidation catalyst.
2. Prior Art
There has been a well-known process for producing acrylonitrile or methacrylonitrile by ammoxidation of propylene or isobutylene. Recently, as a substitute for such a process using propylene or isobutylene, attention has been attracted to a process for producing acrylonitrile or methacrylonitrile by gaseous phase catalytic ammoxidation of propane or isobutane, i.e., by gaseous phase catalytic reaction of propane or isobutane with ammonia and molecular oxygen.
In the ammoxidation of propane or isobutane, stoichiometrically, the molar amount of the reacted ammonia is equal to the molar amount of the reacted propane or isobutane, namely, the molar ratio of the reacted ammonia to the reacted propane or isobutane is stoichiometrically unity (1). However, generally, during the course of the ammoxidation, ammonia, which is one of the gaseous feedstocks for the ammoxidation, is not only converted to by-products (such as acetonitrile and hydrocyanic acid) as well as acrylonitrile or methacrylonitrile as a desired product, but also is decomposed into nitrogen by oxidation [see Applied Catalysis A General (vol. 157, pp.143-172, 1997)].
That is, the conventional catalysts for use in the ammoxidation of propane or isobutane pose a problem in that, during the ammoxidation, conversion of ammonia to by-products and decomposition of ammonia into nitrogen occur to a large extent, leading to a lowering of the yield of acrylonitrile or methacrylonitrile, not only in terms of the yield based on propane or isobutane but also in terms of the yield based on ammonia (hereinafter, the yield of acrylonitrile or methacrylonitrile, based on the fed propane or isobutane, is frequently referred to as "propane- or isobutane-based yield of acrylonitrile or methacrylonitrile", and the yield of acrylonitrile or methacrylonitrile, based on the fed ammonia, is frequently referred to as "ammonia-based yield of acrylonitrile or methacrylonitrile").
The propane- or isobutane-based yield of acrylonitrile or methacrylonitrile can be increased by a method in which feedstock ammonia is fed in an amount larger than the molar amount of the fed propane or isobutane, that is, the molar ratio of the fed ammonia to the fed propane or isobutane is increased to more than 1. However, needless to say, in this method in which ammonia is simply fed in an excess amount, the ammonia-based yield of acrylonitrile or methacrylonitrile further decreases, that is, the utilization of feed-stock ammonia further decreases. In this connection, it should be noted that the cost of ammonia is usually almost equal to that of propane or isobutane. Therefore, when the amount of the fed ammonia is increased in the ammoxidation of propane or isobutane, the overall cost for producing acrylonitrile or methacrylonitrile by ammoxidation disadvantageously increases.
On the other hand, when the amount of the fed ammonia is decreased, the ammonia-based yield of acrylonitrile or methacrylonitrile can be increased. However, the conventional catalysts have a problem in that a decrease in the amount of the fed ammonia inevitably causes a large decrease in the propane- or isobutane-based yield of acrylonitrile or methacrylonitrile. That is, conventionally, the ammonia-based yield of acrylonitrile or methacrylonitrile cannot be increased without causing a large decrease in the propane- or isobutane-based yield thereof.
Thus, for efficiently and economically producing acrylonitrile or methacrylonitrile from propane or isobutane by ammoxidation, it is very advantageous that conversion of ammonia to by-products and decomposition of ammonia into nitrogen during the ammoxidation are suppressed to a level as low as possible, to thereby increase the ammonia-based yield of acrylonitrile or methacrylonitrile without sacrificing the propane- or isobutane-based yield thereof.
With respect to catalysts and methods for use in the ammoxidation of propane or isobutane, a number of proposals have been made.
For example, as a catalyst for use in producing acrylonitrile or methacrylonitrile from propane or isobutane by ammoxidation, an oxide catalyst containing molybdenum, vanadium, niobium and tellurium are known. Such oxide catalysts are disclosed in U.S. Pat. No. 5,049,692, U.S. Pat. No. 5,231,214, U.S. Pat. No. 5,472,925, Unexamined Japanese Patent Application Laid-Open Specification No. 7-144132, Unexamined Japanese Patent Application Laid-Open Specification No. 8-57319 and Unexamined Japanese Patent Application Laid-Open Specification No. 8-141401.
Further, European Patent Application Publication No. 767 164 A1 discloses an oxide catalyst containing molybdenum, vanadium, antimony and X wherein X is at least one element selected from the group consisting of niobium, tantalum, tungsten, titanium, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, boron, indium, cerium, alkali metals and alkaline earth metals.
Among the above prior art documents, each of U.S. Pat. No. 5,049,692, Unexamined Japanese Patent Application Laid-Open Specification No. 7-144132, Unexamined Japanese Patent Application Laid-Open Specification No. 8-57319 and Unexamined Japanese Patent Application Laid-Open Specification No. 8-141401 also discloses oxide catalysts containing, in addition to molybdenum, vanadium, niobium and tellurium, other types of elements. However, in any of these prior art documents, there is no working example in which an ammoxidation of propane or isobutane is performed using such oxide catalysts containing, in addition to molybdenum, vanadium, niobium and tellurium, other types of elements.
Further, U.S. Pat. No. 5,231,214 discloses an oxide catalyst containing molybdenum, vanadium, niobium, tellurium and at least one element selected from the group consisting of magnesium, calcium, strontium, barium, aluminum, gallium, thallium, indium, titanium, zirconium, hafnium, tantalum, chromium, manganese, tungsten, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, zinc, tin, lead, arsenic, antimony, bismuth, lanthanum and cerium. However, among the above-mentioned elements other than molybdenum, vanadium, niobium and tellurium, the elements used in the catalysts prepared in the working examples of this prior art document are only manganese, nickel, magnesium, iron, tin, cobalt, aluminum, calcium, barium, antimony, bismuth, zinc, tantalum, tungsten, chromium, titanium and palladium.
The oxide catalysts disclosed in all the above prior art documents are disadvantageous not only in that a satisfactory level of propane- or isobutane-based yield of acrylonitrile or methacrylonitrile cannot be achieved, but also in that the ammonia-based yield of acrylonitrile or methacrylonitrile is not satisfactory.
On the other hand, U.S. Pat. No. 5,472,925 discloses two types of catalysts. Specifically, one type of catalyst is an oxide catalyst [hereinafter, frequently referred to as "catalyst (A)"] comprising a compound oxide containing molybdenum, vanadium, tellurium and X (wherein X is at least one element selected from the group consisting of niobium, tantalum, tungsten, titanium, aluminum, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony, bismuth, boron and cerium). The other type of catalyst is an oxide catalyst [hereinafter, frequently referred to as "catalyst (B)"] which is obtained by a method in which a compound containing at least one element selected from the group consisting of antimony, bismuth, cerium, boron, manganese, chromium, gallium, germanium, yttrium and lead is added to and mixed with the compound oxide of catalyst (A) above.
In this prior art document, there is a description of the ammoxidation of propane or isobutane using, as catalyst (A) mentioned above, an oxide catalyst containing molybdenum, vanadium, niobium and tellurium. By this prior art technique, when the molar ratio of the fed ammonia to the fed propane or isobutane (hereinafter, frequently referred to as "[ammonia:propane or isobutane] molar ratio") is 1 or less, the ammonia-based yield of acrylonitrile or methacrylonitrile is improved. However, this technique is disadvantageous not only in that the improvement in the ammonia-based yield of acrylonitrile or methacrylonitrile is unsatisfactory, but also in that the propane- or isobutane-based yield of acrylonitrile or methacrylonitrile markedly lowers when the [ammonia:propane or isobutane] molar ratio is 1 or less.
The above prior art document also has descriptions of the ammoxidations of propane or isobutane using, as catalyst (B) mentioned above, a catalyst comprising a mixture of diantimony tetraoxide (Sb.sub.2 O.sub.4) and a compound oxide containing molybdenum, vanadium, tellurium and niobium. In some of these ammoxidations, even when the [ammonia:propane or isobutane] molar ratio is 1 or less, the propane- or isobutane-based yield of acrylonitrile or methacrylonitrile is improved. In some of these ammoxidations, when the [ammonia:propane or isobutane] molar ratio is 1 or less, although a lowering of the propane- or isobutane-based yield of acrylonitrile or methacrylonitrile occurs, the ammonia-based yield of acrylonitrile or methacrylonitrile is remarkably improved.
However, this prior art technique has a disadvantage in that, for obtaining the above-mentioned catalyst (B), it is necessary to employ a complicated and cumbersome production method. Specifically, the catalyst production method comprises: preparing a compound oxide containing molybdenum, vanadium, tellurium and niobium; molding the prepared compound oxide into a tablet; subjecting the obtained tablet to pulverization and sifting, to thereby obtain a particulate compound oxide; subjecting the obtained particulate compound oxide to calcination under a stream of nitrogen gas; grinding the resultant calcined, particulate compound oxide by means of a mortar to obtain a ground compound oxide; adding diantimony tetraoxide (Sb.sub.2 O.sub.4) to the ground compound oxide, to thereby obtain a mixture; molding the obtained mixture into a tablet; subjecting the resultant tablet to pulverization and sifting, to thereby obtain a particulate catalyst precursor; and subjecting the obtained catalyst precursor to calcination under a stream of nitrogen gas, to thereby obtain a catalyst (B). Thus, this catalyst (B), which requires such burdensome production method, is disadvantageous from the commercial viewpoint.
Therefore, it has been strongly desired to develop an improved ammoxidation catalyst which is advantageous not only in that the ammonia-based yield of acrylonitrile or methacrylonitrile can be largely increased without sacrificing the propane- or isobutane-based yield of acrylonitrile or methacrylonitrile, but also in that the catalyst can be-easily produced and hence is advantageous from the commercial viewpoint.